BAIAE, an ancient city of Campania, Italy, 10 m. W. of Neapolis, on the Sinus Baianus, a bay on the W. coast of the Gulf of Puteoli. It is said to derive its name from Βαῖος, the helmsman of Ulysses, whose grave was shown there; it was originally, perhaps, the harbour of Cumae. It was principally famous, however, for its warm sulphur springs, remarkable for their variety and curative properties (Pliny, Hist. Nat. xxxi. 4), its mild climate, and its luxuriant vegetation (though in summer there was some malaria in the low ground). It was already frequented, especially by the rich, at the end of the republican period; and in Strabo's day it was as large as Puteoli. Julius Caesar possessed a villa here, the remains of which are probably to be recognized in some large substructures on the ridge above the 16th-century castle. Baiae was a favourite residence of the emperors. Nero built a huge villa probably on the site now occupied by the castle. Hadrian died in Caesar's villa in A.D. 138, and Alexander Severus erected large buildings for his mother. Baiae never became, however, an independent town, but formed part of the territory of Cumae. Three glass vases with views of the coast and its buildings were published by H. Jordan in Archäologische Zeitung (1868, 91). The luxury and immorality of the life of Baiae under both the republic and the empire are frequently spoken of by ancient writers.

Near Baiae was the villa resort of Bauli, so called from the Βοαύλια (stalls) in which the oxen of Geryon were concealed by Hercules. By some it is identified with the modern village of Bacoli (owing to a presumed similarity to the ancient name), 2 m. S.S.E. of Baiae; by others with the Punta dell' Epitaffio, 1 m. N.E. of Baiae (see G. B. de Rossi in Notizie degli scavi, 1888, 709). At Bauli, Pompey and Hortensius possessed villas, the former on the hills, while that of the latter, on the shores of the Lacus Lucrinus, was remarkable for its tame lampreys and as the scene of the dialogue in the second book of Cicero's Academica Priora; it afterwards became imperial property and was the scene of Agrippina's murder by Nero. It was from Bauli to Puteoli that Caligula built his bridge of boats.

Of the once splendid villas and baths of Baiae and its district, the foundations of which were often thrown far out into the sea, considerable, though fragmentary, remains exist. It is not, as a rule, possible to identify the various buildings, and the names which have been applied to the ruins are not authenticated. At Baiae itself there exist three large and lofty domed buildings, two octagonal, one circular, and all circular in the interior, of opus reticulatum and brick, which, though popularly called temples, are remains of baths or nymphaea. The Punta dell' Epitaffio also is covered with remains, while at Bacoli are several ruins—to the north of the village a small theatre, called the tomb of Agrippina; under the village the remains of a large villa; to the E. the remains of a large water reservoir, the so-called Cento Camerelle; to the S. another with a vaulted ceiling, known as the piscina mirabilis, measuring 230 by 85 ft. The villa of Marius, which was bought by Lucullus, and afterwards came into the possession of the imperial house, was the scene of the death of Tiberius. It is sometimes spoken of as Baiana, sometimes as Misenensis, and is perhaps to be sought at Bacoli (Th. Mommsen in Corp. Inscrip. Latin., x., Berlin, 1883, 1748), though Beloch inclines to place it on the promontory S. of Misenum, and this perhaps agrees better with the description given by Phaedrus.

Baiae was devastated by the Saracens in the 8th century and entirely deserted on account of malaria in 1500.